Mandril assembly for chemical injection in oil wells

ABSTRACT

The present invention refers to a mandrel assembly for chemical injection in use in an oil well production string where the water depth can reach 3,000 m, exerting extreme hydrostatic pressures on the mandrel. The mandrel assembly is capable of dosing the chemical fluid flow rate to avoid some types of unwanted situations in the production string, mainly related to fouling. It has the characteristic of not using injection valves having small diameters, causing the annular space between the mandrel body and the inner part to ensure greater space for the flow of chemical fluid, thus reducing the likelihood of a possible plugging. Such characteristics ensure lower maintenance interventions, thus generating lower costs and increasing well reliability.

FIELD OF INVENTION

The technology of the concentric chemical injection mandrel assembly maybe used in the area of lift and flow of Pre-Salt and Post-Salt wells, toreplace mandrels having state-of-the-art chemical injection valve thatare installed in production strings of the wells.

DESCRIPTION OF THE STATE-OF-THE-ART

A chemical injection mandrel is generally used to pump chemicals intothe production string of wells, aiming at dosing specific chemicals toavoid some types of problems such as corrosion inhibitors, H₂S (sulfidegas) inhibitors and salt scale inhibitors, such as BaSO₄ (bariumsulfate) SrSO₄ (strontium sulfate) and others.

Mandrel is equipment that is normally assembled as an integral part ofthe production string. Attached to it, the hydraulic lines, which arefixed to the string by clamps, are used to transport chemicals from UEP(stationary production unit) to ANM (wet Christmas tree). The hydrauliclines, from the ANM, extend to the specific depth at which the mandrelis positioned in the production string of the well.

The mandrels are equipped with chemical injection valves that areconnected to the hydraulic lines, these chemical injection valvesinterface between the mandrel and the string, usually they have smallinternal diameter (ID) around ⅛″, this small internal diameter of theinjection valve allows its obstruction with ease.

Obstruction can occur by small pieces of metal material inside theflocks, precipitates resulting from the quality of the chemicalmaterials being pumped, dirt resulting from the lack of effectivefiltration of the fluids to be pumped, which require a special class offiltration referred to as NAS 6. This obstruction when it cannot bemitigated leads to loss of functionality of the mandrel and consequentlyloss of injection of the product being dosed into the production string.

Several wells have lost the operation of the chemical injection mandreland consequently problems related to absence of chemical protection suchas column plugging by fouling of inorganic salts, production lossesassociated with reduction of inner diameter of the column by inorganicfouling, loss of H₂S inhibition and etc.

Another problem that is currently under discussion is string breakageduring scale inhibitor pumping, which interferes with inhibitor dosing.This phenomenon occurs when the liquid column formed within thehydraulic line creates a hydrostatic pressure which, when this columnreaches a certain pressure, occurs a discharge of liquid from the insideof the line by the chemical injection valve.

The occurrence of this phenomenon affects reservoir management, as thedosage flow rate is calculated according to laboratory tests, where thepercentages of inhibitor to be dosed are defined in relation to theconcentration of salts present in water that is produced along with theoil, thus, after discharge, it takes a time to stabilize the flow rateagain. During this interval, the well is not adequately inhibitedthereby leading to scale formation in the production string of the well.

When the valve of a mandrel fails, shortly thereafter there is foulingin the production string and therefore reduction of the diameter of thestring, thus creating a choke that reduces the oil production flowreaching UEP.

To solve the loss of production, it will be necessary to remove thescale in the string, requiring the production shutdown of the well.Depending on the project, it will also be necessary to use a completionrig, which is a critical resource and with high cost. In certainoccasions there is the need for the use of stimulation vessels to pumpscale removing chemicals, another critical and expensive resource. Andit may also happen to use the intervention rig along with thestimulation vessel.

Document BR1020120104261A2 discloses a mandrel for mitigating productionlosses associated with plugging due to fouling in production strings offixed platforms where depth is around 100 m. Unlike the invention thatoperates in wells in deep water depths (2,000 and 3,000 m) with extremehydrostatic pressures, presenting various interfaces with subseaequipment, such as ANM. However, it has an injection ring having microvalves, which causes clogging.

Document EP2976495B1 discloses a chemical solution injection mandrelcapable of inhibiting scale in the production string of an oil well.Unlike the invention, the document uses injection valves, such valvesare the main reason for clogging.

Document US20040084186A discloses an apparatus and methods for releasingdownhole well treatment chemicals. It provides a method for passivecontinuous release of well treatment chemicals inhibiting crustformation in the production string. Unlike the invention, the documentuses the release/dissolution of the impregnated material in acylindrical tube fixed to the column (external or internal) by theproduced water.

The state of the art cited above does not have the unique features thatwill be presented in detail below.

BRIEF SUMMARY OF THE INVENTION

The present invention refers to a mandrel assembly for chemicalinjection to be used in an oil well production string where the waterdepth can reach 3,000 m, exerting extreme hydrostatic pressures on themandrel.

The mandrel assembly is capable of dosing the flow of chemicals to avoidsome types of unwanted situations in the production string, such asfouling. It has the characteristic of not using injection valves,causing the annular space between the mandrel body and the inner part toensure greater space for the flow of chemical fluid, thus reducing thelikelihood of a possible plugging. These characteristics guarantee lowermaintenance interventions, thus generating lower costs.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will be described in more detail hereinafter withreference to the accompanying drawings which, in a schematic andnon-limiting manner of the inventive scope, depict exemplary embodimentsthereof. In the drawings, we have:

FIG. 1 illustrates the prior art chemical injection mandrel installed onthe side of the production string

FIG. 2 illustrates the prior art chemical injection mandrel in detail

FIG. 3 illustrates the mandrel assembly of present invention

FIG. 4 illustrates all coupled parts of the mandrel assembly

FIG. 5 illustrates the concentric jacket and its external thread

FIG. 6 illustrates the detail of the pressurization device;

FIG. 7 illustrates the injection of inhibitor fluid.

DETAILED DESCRIPTION OF THE INVENTION

The invention mandrel assembly (2) is comprised of three main parts,namely: concentric mandrel tube (100), concentric jacket (200) andchemical injection mandrel (300), as can be seen in FIG. 3 . The threeparts comprise a single assembly, which although the nature of thedesign thereof allows to produce them independently, each part allows tofacilitate the installation thereof in the exploitation and productionwell, in addition to being adapted to solve the recurrent technicalproblem of plugging the chemical injection lines of the prior art (1).

The concentric mandrel (100) depicted in FIG. 3 for chemical injectionconsists of three parts: the first upper part of the mandrel tube (101)has a female thread (106) and a parallel male thread (102) for couplingthe cap (202). The second part, the central part of the mandrel tube(103) comprises the mandrel body receiving the concentric jacket (201)and thus comprises an annular part (3). In addition, it has side holesreferred to as access devices (104), existing for the chemical to beinjected into the production string. And the third, the lower part ofthe tube (105), preferably comprises a tapered male pin thread, but maybe of the parallel male type.

A chemical injection mandrel (300) is basically comprised of a partcontaining a hydraulic engagement connector (302) of the injectionmandrel (300) with the hydraulic line (301) and another part where thedevices are inserted, being rupture disc (303) 2 check valves (304), anda pressurization device (305).

The concentric jacket (200) is made to be engaged with the concentricmandrel (100), preferably via parallel thread (203) with the parallelmale thread (102), being types of threads that can ensure sealing of thechemical fluid from the annulus (3). This connection between theconcentric jacket (200) and the concentric mandrel (100) forms anannulus (3), purposely placed to allow passage of the fluid into theproduction string. The connection between the annulus and the productionstring is made via access devices (104), central part of the concentricmandrel (100). The number of holes and the size thereof varies accordingto the required flow rate, column diameter and the need to couple apressure valve (107).

The chemical injection mandrel (300) may be inserted directly into thesolid portion (201) of the concentric jacket (200). The amount ofthrough-holes in the solid part (201) allows placing a certain amount ofchemical injection mandrel (300). This allows a concentric jacket (200)to have different amounts of chemical injection mandrel (300) for agiven well, allowing for flexibility of design. Another way is tosegregate the solid part (201) of the concentric jacket (200) and thushave a type of cap (202) having external thread (204), keeping theinternal thread (203), which will hold onto the concentric mandrel body(100); and the external thread of the cap (204) will hold onto the wallof the concentric jacket (200) of its internal side. Likewise, the capmay contain a number of holes (205) according to the well design.

The advantage of the concentric jacket (200) is that the parts of theold chemical injection valve are already mounted in the solid space(201) intended for drilling or in the cap (202) of the concentric jacket(200) with holes (205) already predetermined. They increase the diameterof the components increasing the passage of the injection fluideliminating all ¼″ to ⅛″ connections and ⅛″ line segments that causedplugging during injection, and at the same time have directcommunication with the annular part (3) formed between the central partof the tube (4) and the concentric jacket (200). Thus, the system willbe less vulnerable to plugging by small particles.

Unlike the prior art chemical injection mandrel, the present inventionbrings the inclusion of a device for hydrostatic column equilibriumknown as pressurization device (305), preventing breakage or spillage ofall column liquid during injection of scale inhibiting fluids.

The holes (205), which may be in the cap (202) or in the solid part ofthe concentric jacket (200), have spaces in which pressurization devices(305) are to be housed, which are provided with springs (306) with thefunction of avoiding column breakage during injection of scaleinhibiting fluids. The pressurization device (305) creates anintermediate pressure chamber between the pressure of inlet point (309)(chemical injection line pressure) and the pressure of outlet point(310) (reservoir static pressure or flow pressure in the annuluscolumn-well production casing). The pressurizing device (305) has ahydrostatic pressure sensor (307,308) and a pressure control (311).

FIG. 4 shows the assembly of the mandrel assembly being carried out byscrewing the concentric jacket cap (200), where there is creation of theannulus (3) in place of the prior art valve, and thus increasing thediameters of the components of the chemical injection mandrel (300),such feature ultimately reduces the risk of plugging in chemicalinjection valves.

The technical advantages will be ensuring scale management, maintainingthe production of wells due to chemical inhibition of scale and the lifeof the mandrel assembly (2). The economic advantages will be theguarantee of the recovery factor projected for the field through themaintenance of production, in addition to the fact that the costsrelated to critical resources, such as stimulation vessels and/or rigsare not necessary for the removal of scale from the production system;such as production strings, production lines, in surface equipment atUEP.

The inhibiting fluid will come from UEP through hydraulic lines (301),and pass through the rupture disc (303). This rupture disc, which iscalibrated to break at a given pressure value, will be used to testhydraulic lines with pressure after mandrel installation. The fluid willfollow and go through two check valves (304) that have the function ofpreventing the production of the well from occurring through thehydraulic lines (301). Check valves (304) are safety valves to preventso-called blowout (uncontrolled oil production) by the chemicalinjection line. The inhibiting fluid will follow the pressurizationdevices (305) of the lines, which have the function of keeping theinhibiting fluid line always full (from UEP to mandrel), then the fluidwill pass through the annulus (3), formed between the inner part of theconcentric jacket (200) and the outer wall of the concentric tube (100),to the access device (104) of the mandrel to the inside of string.

In the access device (104), there may be installed a kind of pressurevalve (107), which will be capable of being regulated/calibrated, to beopened with a certain pressure value applied, from its manufacturingdesign to work with positive pressure, to be calculated as a function ofthe hydrostatic pressure (calculated by the weight of the inhibitingfluid at the vertical depth of TVD design), considering the highestinhibiting fluid density value to be used in the design. Thus, theinjection of chemicals into the inside of string will occur when thepressure value inside the mandrel overcomes a minimum value, driven by apump located in UEP. This way ensures a stable and constant inhibitingfluid injection hydraulic column.

In FIG. 7 it is possible to notice the inhibiting fluid (5) coming fromfilling the hydraulic line (301) traveling through the annulus (3) andbeing injected by the access device (104).

The number of access devices (104) will depend on the design, dependingon the column diameter, 3½″, 4½″, 5½″ and 6⅝″. Thus, depending on thedesign, the amount of devices can be increased or decreased. It may alsobe due to the flow rate of the production string or the use of apressure valve (107).

However, the assembly of the mandrel assembly (2) in the productionstring is facilitated, once the concentric mandrel tube (100) is fittedto the string, being enough to displace the concentric jacket (200),which has a larger internal diameter than the production string, untilreaching the threads (102) of the concentric tube (100). In addition,the various chemical injection mandrels (300) are already pre-installedin the concentric jacket (200) or cap (202), avoiding detailedassemblies during installation in the well string. An unused chemicalinjection hydraulic line (301) may be replaced by another as there willbe no return of the produced oil, increasing the reliability of thewell.

The invention claimed is:
 1. A chemical injection mandrel assembly for ahydrostatic pressure well comprising: a concentric mandrel tubeincluding an upper portion, a central portion, and a lower portion, theupper portion having a male thread and the central portion including oneor more access holes; a concentric jacket including a solid portionincluding one or more holes and an internal female thread configured tocouple the concentric jacket with the concentric mandrel tube byengaging the male thread, the coupling of the concentric jacket and theconcentric mandrel tube forming an annulus covering the central portionof the concentric mandrel tube; and a chemical injection mandrelincluding a pressurization device, the chemical injection mandrel beinginserted in one of the one or more holes in the solid portion of theconcentric jacket such that the pressurization device is housed withinthe holes of the solid portion of the concentric jacket, wherein theannulus allows for passage of fluid from the chemical injection mandrelto a production string of the hydrostatic pressure well via the one ormore access holes of the concentric mandrel tube.
 2. The chemicalinjection mandrel assembly according to claim 1, wherein the chemicalinjection mandrel further includes a hydraulic connector, a rupturedisc, and two check valves.
 3. The chemical injection mandrel assemblyaccording to claim 1, wherein the concentric jacket further includes acap and the internal female thread and solid portion of the concentricjacket are part of the cap.
 4. The chemical injection mandrel assemblyaccording to claim 1, wherein the pressurization device includes aspring.
 5. The chemical injection mandrel assembly of claim 4, whereinthe pressurization device creates an intermediate pressure chamberbetween a pressure of the chemical injection line and a reservoir staticpressure or flow pressure in the production string.
 6. The chemicalinjection mandrel assembly of claim 5, wherein the pressurization devicefurther includes a hydrostatic pressure sensor and a pressure control.7. The chemical injection mandrel assembly according to claim 1, furthercomprising a pressure valve in each of the one or more holes of thecentral portion of the concentric mandrel tube.
 8. The chemicalinjection mandrel assembly of claim 7, wherein the each pressure valveis capable of being regulated/calibrated to work with positive pressureat a true vertical depth at which the production string of thehydrostatic pressure well is installed.
 9. The chemical injectionmandrel assembly of claim 8, wherein the positive pressure is calculatedas a function of a hydrostatic pressure of a highest density of asubstance contained in an inhibitory fluid mixture.
 10. The chemicalinjection mandrel assembly according to claim 7, wherein the pressurevalve starts injection into the production string when a pressure valueinside the mandrel overcomes a preset minimum value, given a dischargepressure of a pump installed in a stationary production unit.
 11. Thechemical injection mandrel assembly of claim 1, wherein a number ofaccess holes in the central portion of the concentric mandrel tube is asa function of a diameter of the production string, a production flowrate of the hydrostatic pressure well or a need of the pressure valvecontained within the one or more holes.
 12. The chemical injectionmandrel assembly according to claim 1, wherein the concentric tubefurther includes a female thread at the upper portion.
 13. The chemicalinjection mandrel assembly according to claim 1, wherein the concentrictube further includes a male conical or male parallel thread at thelower portion.